This invention relates to a method and apparatus for cleaning a plurality of filter elements which are vertically positioned within a filter vessel.
The method and apparatus in accordance with the invention is disclosed in conjunction with a specific filter apparatus which utilizes pre-coat type filter elements. It is contemplated that the invention may be similarly utilized in conjunction with other types of filter apparatus including filter apparatus which utilize non-pre-coat type filter elements.
It is known that liquids may be very effectively purified by passing them through a filter screen which has been pre-coated with a layer of particles, such as ion exchange resin particles. This method is disclosed in U.S. Pat. No. 3,250,703, issued May 10, 1966, and assigned to the same assignee of this application. Periodically, the layer of particles becomes exhausted and a filter cake builds up. It is then necessary to clean the filter in order to remove the particles so that they can be replaced with another layer of fresh particles, referred to herein as a "pre-coat" layer.
The filters normally employed are annular filter elements that are made of perforated metal or the like covered with a screen or with wool, cotton, nylon or other natural or synthetic material to form a relatively fine filter. The filter elements are typically positioned within a filter vessel and are secured to a tube sheet which divides the filter vessel into a filter compartment and a plenum compartment. There are generally two types of such filter apparatus which employ vertically positioned elements; one is of the down flow service water type which has the plenum compartment above the filter compartment, an example of such type is disclosed in U.S. Pat. No. 3,637,079, and the other is of the up flow service water type which has the plenum compartment below the filter compartment, an example of such type is disclosed in U.S. Pat. No. 3,666,097. The present invention is particularly directed to cleaning the filter elements in the latter type of filter apparatus.
It is the well known practice to clean the filter elements in a filter vessel of the up flow service type by use of a series of steps which include a series of backwash cycles which force a flow of cleaning liquid through the filter elements in the reverse direction of the service flow at high velocities. The high velocity may be imparted to the cleaning liquid by use of gas from a pressurized source. Such a backwash cycle is commonly known as a gas or air surge backwash cycle. The volume of cleaning liquid which is forced through the filter elements during each backwash cycle is approximately equal to the volume of cleaning liquid which is in the plenum compartment above the lower ends of the tube sheet fittings associated with the filter elements. In existing filter apparatus where the volume of the plenum area is fixed, and in newly designed filter apparatus where limitations are placed on the vessel length or tube sheet fitting length, the volume of cleaning liquid flow during each cycle is accordingly limited.
The present invention is based on the problem of finding a method and apparatus for cleaning filter elements which are supported within vessels having a bottom plenum volume which is not sufficient to contain the desired volume of cleaning liquid.